Extended heat-transfer relation for an impinging laminar flame jet to a flat plate

Many industrial applications use flame impingement to obtain high heat-transfer rates. An analytical expression for the convective part of the heat transfer of a flame jet to a plate is derived. Therefore, the flame jet is approximated by a hot inert jet. In contradiction with existing convective heat-transfer relations, our analytical solution is applicable not only for large distances between the jet and the plate, but also for close spacings. Multiplying the convective heat transfer by a factor which takes chemical recombination in the cold boundary layer into account, results in an expression for the heat flux from a flame jet to the hot spot of a heated plate. Numerical and experimental validation show good agreement.     

Optimization principles for convective heat transfer

Optimization for convective heat transfer plays a significant role in energy saving and high-efficiency utilizing. We compared two optimization principles for convective heat transfer, the minimum entropy generation principle and the entransy dissipation extremum principle, and analyzed their physical implications and applicability. We derived the optimization equation for each optimization principle. The theoretical analysis indicates that both principles can be used to optimize convective heat-transfer process, subject to different objectives of optimization. The minimum entropy generation principle, originally derived from the heat engine cycle process, optimizes the convective heat-transfer process with minimum usable energy dissipation focusing on the heat–work conversion. The entransy dissipation extremum principle however, originally for pure heat conduction process, optimizes the heat-transfer process with minimum heat-transfer ability dissipation, and therefore is more suitable for optimization of the processes not involving heat–work conversion. To validate the theoretical results, we simulated the convective heat-transfer process in a two-dimensional foursquare cavity with a uniform heat source and different temperature boundaries. Under the same constraints, the results indicate that the minimum entropy production principle leads to the highest heat–work conversion while the entransy dissipation extremum principle yields the maximum convective heat-transfer efficiency.     

Influence of glass curtain walls on the building thermal energy consumption under Tunisian climatic conditions: The case of administrative buildings

The glass curtain walls have been recently introduced in Tunisia; they are seen as a new fashion and are highly appreciated by some for their pleasing aesthetics. The objective of this paper is to investigate whether the glass curtain walls are appropriate for the Tunisian local climate and context and if it is so, to give recommendations concerning the kind of glass to be used.A TRNSYS [Klein SA. TRNSYS a transient system simulation Program V5 14.2. Solar Energy Laboratory, University of Wisconsin Madison, July 1996] simulation was conducted on a typical administrative building. The investigation concerns only the building heating and cooling load. The building was split in five thermal zones; for each thermal zone, all the windows have the same orientation. The single zone model TYPE19 of TRNSYS [Klein, 1996] was used to model each thermal zone. An additional convection heat transfer between the different thermal zones of the building was modelled according to the Brown and Solvason law [Brown WG, Solvason KR. Natural convection through rectangular openings in partitions, Part 1: vertical partitions. Int. J. Heat Mass Transfer 1962; 5: 859–68]. This particular law was used because it has been validated in the Tunisian context by Bouden [Bouden C. Analyse du suivi thermique d’un pavillon solaire expérimental en région Tunisoise, Thèse de doctorat, Université de Paris 7, 1989]. We assume that the glass curtain wall will be implemented only on the main building facade; this is why it was simulated with different glazing sizes and glass types. The other facades remain unchanged. The results of this simulation have shown that, in relation to space heating, the glass curtain wall can be very interesting in the Tunisian context if the orientation as well as the kind of glazing are carefully selected.     

Inverse influence of initial diameter on droplet burning rate in cold and hot ambiences: a thermal action of flame in balance with heat loss

Isolated droplet burning were conducted in microgravity ambiences of different temperatures to test the initial diameter influence on droplet burning rate that shows a flame scale effect and represents an overall thermal action of flame in balance with heat loss. The coldest ambience examined was room air, which utilized a heater wire to ignite the droplet. All other ambiences hotter than 633 K were acquired through an electrically heated air chamber in a stainless steel can. An inverse influence of initial droplet diameter on burning rate was demonstrated for the cold and hot ambiences. That is, the burning rate respectively decreased and increased in the former and latter cases with raising the initial droplet diameter. The reversion between the two influences appeared gradual. In the hot ambiences the burning rate increase with increasing the initial droplet diameter was larger at higher temperatures. A “net heat” of flame that denotes the difference between “heat gain” by the droplet and “heat loss” to the flame surrounding was suggested responsible for the results. In low-temperature ambiences there is a negative net heat, and it turns gradually positive as the ambience temperature gets higher and the heat loss becomes less. Relating to luminous flame sizes and soot generation of differently sized droplets clarified that the flame radiation, both non-luminous and luminous, is determinative to the net heat in microgravity conditions. In addition, the work identified two peak values of soot generation during burning, which appeared respectively at the room temperature and at about 1000 K. The increase in ambience temperature made also bigger soot shells. The heat contribution of flame by both radiation and conduction was demonstrated hardly over 40% in the total heat required for droplet vaporization during burning in a hot ambience of 773 K.     

Solar-heated-air receivers

Receiver design alternatives for a central tower, heat-air receiver of a solar/gas turbine electrical generation plant are considered. Apertured and unapertured, domed-surface and -cavity receivers are examined and losses such as incident flux reflection and reradiation from the receiver are included. The receiver, constructed of ceramic domes that are individually cooled by impingement-jet heat-transfer techniques, is designed to supply heated air at 1800°F and operate in a pressurized condition at a pressure ratio of four. It is shown that high thermal conversion efficiencies (80–90%) may be achieved with cavity receivers where the interior cavity surfaces are formed from single or multiple domes. The efficiencies of surface receiver elements are found to be substantially less than those of cavities, from 54 to 70 per cent. The difference lies in the higher reradiation flux losses of surface receivers.     

Multi-Objective Optimization of Micro Pin-Fin Arrays for Cooling of High Heat Flux Electronics with a Hot Spot

ABSTRACT

This article presents fully three-dimensional conjugate heat transfer analysis and a multi-objective, constrained optimization to find sizes of pin-fins, inlet water pressure, and average speed for arrays of micro pin-fins used in the forced convection cooling of an integrated circuit with a uniformly heated 4 × 3 mm footprint and a centrally located 0.5 × 0.5 mm hot spot. Sizes of micro pin-fins having cross sections shaped as circles, symmetric airfoils, and symmetric convex lenses are optimized to completely remove heat due to a steady, uniform heat flux of 500 W cm^?2 imposed over the entire footprint (background heat flux) and a steady, uniform heat flux of 2000 W cm^?2 imposed on the hot spot area only (hot spot heat flux). The two simultaneous objectives are to minimize maximum substrate temperature and minimize pumping power, while keeping the maximum temperature constrained below 85°C and removing all of input thermal energy by convection. The design variables are the inlet average velocity and size of the pin-fins. A response surface is generated for each of the objectives and coupled with a genetic algorithm to arrive at a Pareto frontier of the best trade-off solutions. Numerical results show that, for a specified maximum temperature, optimized arrays with pin-fins having symmetric convex lens shapes create the lowest pressure drop, followed by the symmetric airfoil and circular cross-section pin-fins. An a posteriori three-dimensional stress–deformation analysis incorporating hydrodynamic and thermal loads shows that Von-Mises stress for each pin-fin array is significantly below the yield strength of silicon, thus, confirming structural integrity of such arrays of micro pin-fins.     

Combined forced and free convection for laminar flow in horizontal tubes with uniform heat flux

This study considers the effects of free convection on laminar flow of water in horizontal circular tubes having essentially constant heat flux at the tube wall. A visual and quantitative study was performed utilizing electrically heated glass tubing. These data were combined with other data and correlations to obtain a general picture of the influence of free convection on the Nusselt number. The final correlation curves given in Fig. 11 are provisionally recommended for obtaining heat-transfer coefficients in practical situations. With reasonable heating rates, the heat-transfer coefficients can be three to four times the values predicted by traditional constant property solutions.     

Formation of a dry spot in a horizontal liquid film heated from below

This study deals with the formation of a dry spot in a non-boiling thin film of ethanol on a horizontal surface upon slowly increasing the heat flux from an embedded nichrome strip. Appreciable thinning of the film occurred prior to rupture, and is associated with the appearance of Bénard-type convective cells. The threshold heat flux for appearance of a dry spot is greater than for disappearance, presumably due to contact angle hysteresis and/or the temperature gradients in the heater strip in the vicinity of the triple interface. A quasi-static stability analysis is given, based upon the equilibrium shape of a semi-infinite drop on a heated surface.     

Analysis of the heat transfer of an impinging laminar flame jet

Flame jet impingement is used in many industrial processes. In this paper an analytical expression is derived for the heat flux of a laminar flame impinging on a flat plate, where the flame jet is approximated by a hot inert jet with the position of the tip of the flame taken equal to the nozzle position. The heal flux in this expression is dependent on the nozzle-to-plate spacing, in contradiction to existing (semi-analytical) relations. The geometry is divided in a region far from the plate and a region dose to the plate. For both regions the velocity profiles are calculated using only the dominant terms of the balance equations. Subsequently these profiles are linked to each other at the boundary between the two zones. Implementing the resulting velocity profile for the complete geometry in the energy equation and integrating over the whole domain results in an expression for the heat flux from the flame to the plate at the hot spot. Numerical calculations show very good agreement with the results of the analytical derivation.     

Effect of flow field for colorless distributed combustion (CDC) for gas turbine combustion

Colorless distributed combustion (CDC) investigated here is focused on gas turbine combustion applications due to its significant benefits for, much reduced NOx emissions and noise reduction, and significantly improved pattern factor. CDC is characterized by distributed reaction zone of combustion which leads to uniform thermal field and avoidance of hot spot regions to provide significant improvement in pattern factor, lower sound levels and reduced NOx emission. Mixing between the combustion air and product gases to form hot and diluted oxidant prior to its mixing with the fuel is critical so that one must determine the most suitable mixing conditions to minimize the ignition delay. Spontaneous ignition of the fuel occurs to provide distributed reaction combustion conditions. The above requirements can be met with different configuration of fuel and air injections with carefully characterized flow field distribution within the combustion zone. This study examines four different sample configurations to achieve colorless distributed combustion conditions that reveal no visible color of the flame. They include a baseline diffusion flame configuration and three other configurations that provide conditions close to distributed combustion conditions. For all four modes same fuel and air injection diameters are used to examine the effect of flow field configuration on combustion characteristics. The results are compared from the four different configurations on flow field and fuel/air mixing using numerical simulations and with experiments using global flame signatures, exhaust emissions, acoustic signatures, and thermal field. Both numerical simulations and experiments are performed at a constant heat load of 25 kW, using methane as the fuel at atmospheric pressure using normal temperature air and fuel. Lower NOx and CO emissions, better thermal field uniformity, and lower acoustic levels have been observed when the flame approached CDC mode as compared to the baseline case of a diffusion flame. The reaction zone is observed to be uniformly distributed over the entire combustor volume when the visible flame signatures approached CDC mode.     

A CFD-derived correlation for methane heat transfer deterioration

ABSTRACT

Methane heat-transfer deterioration can occur in the regenerative cooling channels of future liquid-oxygen/liquid-methane rocket engines with chamber pressures higher than about 50 bar. Aiming to improve the prediction capabilities for the design of such systems, in the present study, a Nusselt number correlation able to describe the convective heat-transfer characteristics of supercritical flow exhibiting deterioration and with negligible buoyancy effects is obtained using data from numerical simulations. The adopted numerical solver of the Navier–Stokes equations is first validated against the experimental data of near-critical hydrogen in heated tubes and then used to collect heat-transfer data of supercritical methane in a heated tube for different levels of pressure, temperature, and mass flux.     

Thermal stresses in radiant tubes due to axial,circumferential and radial temperature distributions

This paper presents an analysis of the thermal stresses in radiant tubes. The analytical analysis is verified using a finite element model. It was found that axial temperature gradients are not a source of thermal stresses as long as the temperature distribution is linear. Spikes in the axial temperature gradient are a source of high thermal stresses. Symmetric circumferential gradients generate thermal stresses, which are low as compared to the stress rupture value of radiant tubes. Radial temperature gradients create bi-axial stresses and can be a major source of thermal stress in radiant tubes. A local hot spot generates stresses, which can lead to failure of the tube.     

Regime diagrams and characteristics of flame patterns in radial microchannels with temperature gradients

Comprehensive regime diagrams of flame pattern formation in radial microchannels with temperature gradients were drawn based on experimental findings. A premixed methane–air mixture was introduced at the center of microchannels formed by two parallel circular quartz plates that were heated with an external porous burner to create a positive temperature gradient condition in the direction of flow. Combustion behavior in those microchannels at channel widths of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm were experimentally investigated. Regime diagrams of various stable and unstable flame patterns were obtained, confirming that the flame pattern is a strong function of mixture equivalence ratio, inlet mixture velocity, and channel width. Furthermore, some combustion characteristics, such as the rotating frequency of the single pelton-like flame and the triple flame, the radius of the stable circular flame front, and comparison between the major combustion products of the single and double pelton-like flames, were also investigated.     

On critical conditions for thermal explosion of a hot spot

The results on the numerical calculation of critical hot spot thermal explosion conditions for plane slab, cylindrical and spherical symmetry are reported. The data obtained have been presented in the form of a functional relation between the well-known Frank-Kamentsky's criterion δ and the dimensionless temperature of the hot spot θ₀. Evaluation of the errors made in the calculation of the value when making use of the Zinn, Friedman, Boddington and Thomas approximate methods has been carried out and non-conformity between approximate theories and exact calculation with respect to the hot spot explosion mechanism has been explained.     

不同富氧含量玻璃熔窑火焰空间温度场的对比

建立了玻璃熔窑火焰空间温度场的三维数学模型，通过对某日产400t燃油浮法玻璃熔窑火焰空间在三种富氧情况(氧含量分别是24％，27％，30％)下用图像模拟直观的表述出计算结果。模型包括气相流动与传热模型，雾化油滴燃烧的轨道模型，和辐射传热模型。程序采用MS-FORTRAN语言，绘图采用Stanfordgraphic软件。对比结果表明，随着富氧含量的增加，各小炉火焰长度明显缩短，温度显著提升，模拟结果对窑炉设计与富氧燃烧组织有一定的参考价值。     

Updated jet flame radiation modeling with buoyancy corrections

Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.     

Basic natural convection in a vertical porous layer differentially heated from its sidewalls subject to lack of local thermal equilibrium

A vertical porous layer subject to lack of local thermal equilibrium and differentially heated from its sidewalls experiences extremely different conditions when it is heated via a constant heat flux, rather than via a hot temperature imposed on the sidewall. For a start the steady state basic temperatures of the fluid and solid differ, as distinct from the corresponding case of imposed temperature on the sidewall when both fluid and solid temperatures are identical and linear at steady state and the lack of local thermal equilibrium (LaLotheq) can manifest itself at transients or when the basic temperature profiles stated above become unstable. In addition, in the case of heating via a constant heat flux the steady state basic temperatures of the fluid and solid are not only distinct but also nonlinear. The analytical derivation of these nonlinear solutions for the basic natural convection in a vertical porous layer differentially heated from its sidewalls, subject to lack of local thermal equilibrium and heating via a constant heat flux is presented and the results analyzed over a wide range of parameter space. In general it is shown that the lack of local thermal equilibrium destroys the symmetry of the problem via deviatoric terms in the solutions.     

Experimental Study of Deposits Removal in Thermal Desalination Plants by the Thermal Shock Technique

Deposition of salts on heat transfer surfaces in thermal desalination plants can lead to operational failure. Scale removal can occur by applying a thermal shock, which is a sudden decrease in the heating process. The difference in thermal expansion between the heat transfer surface and the deposit layer plays a key role in the thermal shock process. The objective of this research is to determine experimentally the minimum temperature of the heating surface in desalination units, such that the thermal shock is still applicable. The minimum heating temperature is important for minimization of heat losses. An experimental setup has been designed and developed, and it consists of an oil tank in which oil is heated by electrical heaters. The heated oil is circulated by a gear pump to the steam generator, which contains the water to be desalinated, that is, a CaSO₄ solution, at atmospheric pressure. The water is heated and converted into steam by the hot oil leaving the salts behind, that is, the fouling layer, on the tubes of the steam generator. A thermal shock is applied when the asymptotic behavior is approached, such that the flow of the hot oil is suddenly stopped for a short period of time before resuming it again. The minimum heating temperature has been determined for two types of tubes: stainless steel and copper, and at a salt concentration of 2 g/L. The minimum heating oil temperature that allows the applicability of the thermal shock is 130°C when using copper tubes, and 140°C for stainless-steel tubes.     

Calculating heat transfer through windows

To calculate the energy performance of buildings, one must know the heat-transfer characteristics of the windows as functions of environmental variables, such as temperature and wind speed. Window designs are becoming more complex in response to the need for energy conservation. In this paper, we develop a general procedure for calculating the net energy flux through the glazed area of a window composed of an arbitrary number of solid layers. These layers, which may have thin-film coatings, can have any specified solar and thermal radiation properties and enclosed spaces between solid layers can contain either air or other gases. We verified our results by comparing them with experimental measurements of heat flow using a calibrated hot-box.     

Mini-Contact Enhanced Thermoelectric Coolers for On-Chip Hot Spot Cooling

Shrinking feature size and increasing transistor density, combined with the high performance demanded from next-generation microprocessors, have led to on-chip high heat flux “hot spots,” which have emerged as the primary driver for thermal management of today's integrated circuit (IC) technology. This article describes the use of a mini-contact to enhance the cooling flux of a miniaturized thermoelectric cooler (TEC) for on-chip hot-spot remediation. A package-level numerical simulation is developed to predict the on-chip hot spot cooling capability achievable with such a mini-contact enhanced TEC. Attention is focused on the hot-spot temperature reduction associated with variations in mini-contact size and thermoelectric element height, as well as the parasitic effect from the thermal contact resistance introduced by the mini-contact. A preliminary experiment has been conducted to verify the numeric model and to demonstrate the effects of the mini-contact on hot-spot cooling.